My dad has dementia and is fixating on doing/repairing things. Will anything help calm this?

Since you've said he becomes aggressive when offered an alternative or taking a break with his fixation on repairing things, I'd say your only hope is to speak to his doctor about stronger medications to keep him calm and asleep through the night. Obviously, he shouldn't be staying up all night with such obsessive/compulsive behaviors, so a review of his meds is in order now. Once that happens, THEN perhaps he can be offered different projects to keep him occupied.

Best of luck with a difficult situation.

My dad had pretty advanced dementia. He was an electrician for about 100 years and was always busy with projects around his little farm. He badly need “jobs” to keep him occupied in his nursing home. I found that he loved sorting coins, wirenuts, screws and bolts and putting together very simple electrical junction box assemblies. I also found some snap together model cars for little kids that he could do a little. And since he had no short term memory we could start a new “job”an hour later. He’d get pretty worn out which was good and felt like he got his jobs done. Slept better also

KPHolloway, wonder if your Dad would be interesting in watch those HGTV shows that show people rehabbing houses, you could watch with him in the evening.

If you know how to fix things, make it a competition on how you would fix something compared to what is being shown in the TV show, but always let Dad win :) Or if you are not familiar, you can ask Dad to explain it to you. He might enjoy being the instructor, and in turn it will give you extra knowledge.

Long time ago there was a Canadian TV show that was shown here in the States, called "Red Green". It was so very comical as Red Green would explain how he would fix something (usually with duct-tape) or make something out of stuff he had laying around in the garage. My sig-other who was not handy couldn't understand why my Dad and I were rolling on the floor with laughter :) I believe the shows are now on YouTube.

Does he have a medication for calming anxiety? In a different set of circumstances a daily Zoloft calmed my dad’s anxiety well

I agree with Alva's suggestion of trying to distract him with sorting activities is one that has been tested and works to help with Sundowning and burning energy towards the late afternoon and early evening. Maybe he can sort (or pair) nuts and bolts, and screws, "clean" tools, etc. It can be the same stuff each time. Or, you can "break" something (or disassemble) that he can fix or build and just keep using the same item and give it to him over and over. Maybe even Duplos or a Lego set (because they have visual instructions). I wish you much success.

Something that you need to discuss with his doctor. He may require medications. It is always disturbing to have to resort to this as drugs, drug cocktails that work are hard to find, and there is such a fine line to aim for between having someone obsessed versus catatonic. I surely do wish you good luck. Meanwhile try for asking him to "help" with making a simple garden, building something easy (if he won't hurt himself with tools) and etc. It is so difficult for some not to have a "job" and when I was a nurse it so often helped, especially women, if we could set them up with a job helping us (folding wash clothes comes to mind).